Generally, polyimide resins are prepared by condensation polymerization of aromatic tetracarboxylic acid or its derivatives and aromatic diamine or aromatic diisocyanate, and polyimide resins thus obtained have superior heat resistance, chemical resistance, mechanical and electrical properties. Photosensitive aromatic polyimides having such superior properties have been widely used for electronic materials such as semiconductor encapsulant, etc.
However, aromatic polyimides are not appropriate for transparent protection layers or insulation layers for liquid crystal display devices in that they have low transmittance in the visible region, are colored in yellow or brown, and have a relatively high dielectric constant. Epoxy resins or acrylic resin compositions have been widely used as transparent protection layers or insulation layers for liquid crystal display devices. However, when they are processed at the temperature of 250° C. or higher, severe discoloration and film contraction may occur and thus subsequent processing condition may be limited. Therefore, it is preferable that they are used under the temperature of 230° C. or lower in view of their inherent heat resistance.
Accordingly, polyimide materials have been taken into consideration in order to meet heat resistance and transparency simultaneously, and some methods of preparation of polyimides by using aliphatic cyclic acid anhydrides to obtain transparent polyimide coating or film have been reported (Macromolecules, 1994. 27, 1117 and 1993, 26, 4961, Japanese Patent Laid-Open No. H9-95533, Japanese Patent Laid-Open No. 2001-330721).
However, in order to use such transparent polyimide resins as protection layers or insulation layers for liquid crystal display devices practically, the process of minute pattern formation by lithography process, which uses a separate photoresist, should be added after polyimide film formation on substrate composed of glasses, etc.
While the use of photosensitive transparent polyimide resin compositions is advantageous in view of cost reduction since the above-described process can be simplified, the photosensitive transparent polyimide resin composition using trans-1,4-diaminocyclohexane reported in Japanese Patent Laid-Open No. 2002-161136 is practically inapplicable due to too slow photosensitivity.
The inventors of the present invention have reported photosensitive transparent polyimide resin composition in Korean Patent Application No. 10-2002-074070, leaving development margin and photosensitivity to be improved further.